1. FIELD OF THE INVENTION
This invention relates generally to observations of the environment that can be made underwater; and more particularly to use of observations from below a rough water surface to determine and use details of the water surface waves--and, based on such details, various above-surface data.
2. PRIOR ART
Even though most of the earth's surface is covered by water--mainly ocean--and satellites now provide observation of conditions at and above the ocean surface, still these observations do not provide detailed close-up data for the characteristics of the ocean surface, and what is going on in and just above that surface. Such data could be extremely useful in improving interpretation of, for instance, satellite radar data to enhance weather prediction, and also in promoting safer use of the ocean regions by many different kinds of air- and water-craft, and improved operation of various automatic sensor arrays.
It is true that such data can be acquired by manned or unmanned equipment floating on the ocean surface, but stations or vessels carrying such equipment are subject to destructive weather events and interactions with shipping. Submerged apparatus whether bottom-resting, bottom-tethered, or mobile is insulated from these problems, and could acquire such data for either relay (e.g., by cable) to land-based stations for beneficial use, and/or for accumulation while at sea and later batch-wise downloading to land stations, and/or even to enhance operations of the submerged apparatus itself.
In addition to the possibility of positioning special submerged observation apparatus at many places around the world, underwater craft are already deployed around the world continuously in considerable numbers. They are thus potentially positioned to collect a large amount of useful local microdata about water surfaces--and the space above them-- that later could be beneficially employed by correlation with other information (for instance, satellite radar pictures) to enhance the interpretation of that other information as well as operations of the craft. Observational limitations, however, deter effective use of this great potential resource.
Unfortunately submerged underwater apparatus is severely limited in ability to detect conditions at or above the surface, and even ability to receive data communications from above the surface. Light and other electromagnetic radiation are strongly attenuated in passing through water, particularly ocean water; acoustic transmission too is limited in range, and awkward for both observation and data transmission.
Thus for example direct optical observation of the surface from below is inadequate to determine whether the weather or water-surface conditions are satisfactory for operations at or above the surface by small craft etc. Some planned operations may be practical or desirable only if there is no heavy rain, or only when wave action is moderate, or only if wind direction and speed are within some range of desired values. As another example, before moving underwater equipment to the surface it would be desirable to have information suggesting that possible collision hazards might be somehow minimized.
The electromagnetic spectrum provides practical means of observation and communication only in two wavelength ranges: in the blue-green part of the visible spectrum (and even there over only very short distances, such as a few hundred meters) and at so-called "extra low frequencies" (ELF) in the radio spectrum.
Ordinarily such objects as surface craft are not readily localized through observation in the extra-low-frequency part of the spectrum. This is so not only because they normally radiate little in the ELF spectral region, and not only because the level of ambient radiation (which might be thought serviceable for observation by reflected ambient) is low, but primarily because the corresponding wavelengths are on the order of 100,000 kilometers.
Thus over terrestrial distances such radiation is inherently omnidirectional. Acoustics is commonly used for sounding and location of nearby objects, and is far more directional than ELF radio--but still imprecise, due to diffusion of acoustic vibration by the water.
Direct visual observation through the air above the water surface, from underwater positions of a submerged vehicle, is of course normally impossible because of the image-scrambling effect of the water surface. Some underwater equipment commonly is equipped with periscopes, which bypass the limitations of water as a transmission medium and employ observation purely through the air.
It might be supposed that such a device could be used to overcome the difficulties outlined in the foregoing paragraphs. Unfortunately these simple mechanical devices serve only when the equipment is within some meters of the surface.
In such shallow positions the submerged apparatus is already within collision range for large surface craft, and of course the periscope itself is within collision range for all surface craft. Furthermore the operations of underwater equipment are more stable and generally more useful at greater depths.
As to communications, a global system of ELF radio transmissions is maintained for sending messages underwater to either stations or craft. Unfortunately the frequency of 14 these vibrations is so low--and the number of receivers that share the system so great--that only extremely short messages can be provided in any normal transmission interval, and particularly to any one receiver.
High-bandwidth transmissions from above the surface are not receivable by equipment at normal operating depths of, for example, thirty meters or more. For instance signals from a blue-green laser beacon (projected, for example, from a satellite) could be transmitted toward the water--but would be badly and dynamically fragmented, and all the fragments dynamically redirected or scrambled, from a rough water surface.
Underwater equipment would not know in which direction to look, at each instant, to see such signals. Most of the energy in acoustic messages from above the water surface would be reflected by the surface.
Thus it can be seen that the prior art in the area of underwater operations has failed to provide effective capability for acquiring information about conditions and details--and for receiving communications from broadband signal transmitters--at and above the surface.
I wish to mention some earlier technological developments that have never heretofore been associated with the operation of underwater apparatus, or with the problems discussed above. One spans the areas of astronomy and what might be called pseudoastronomy--i. e., visual tracking of satellites.
A second is in the area of satellite-based data collection to analyze conditions of the ocean surface. A third earlier area relates to radar processing.
As to the first, it is known to enhance telescope observations through layers of dynamically shifting atmospheric disturbance, by making correspondingly dynamic compensating adjustments in the focus of the telescope. Focal calibration for this purpose is enabled by a reference source such as a bright star of known position--or an optical beacon, either satellite-based or projected from the ground.
The reference actual or artificial "guide star" must be found or placed angularly very near the object to be observed; and focal compensation (inclusion of the "conjugate" of the atmospherically introduced perturbation) is provided through use of adaptive optics--systems in which a telescope reflector is formed from multiple individually servocontrolled reflecting facets. An anecdotal survey of such work appears at Collins, "Making Stars to See Stars . . . " 45 Physics Today 17-21 (February 1992).
As to the second earlier field of work mentioned above, Cox and Munk have described use of marine-surface observations by airborne or orbiting optical instruments to determine the energy-spectral density of ocean waves. See "Measurements of the Roughness of the Sea Surface from Photographs of the Sun's Glitter", 44 J. Optical Society of America 838-50 (November 1954). Their work used the visual "sky gradient" (angular dependence of apparent brightness of the daytime sky) as a reference, and relied upon a theory of preferential reflectance of certain parts of the sky by nonuniform wave-surface angles to support Fourier-transform calculations leading to the desired energy analysis.
Cox and Munk neither proposed nor suggested any effort to dissect the wave-formed remapping of the sky to learn or use the orientations of, for example, individual waves. They simply performed a mathematical reversal of the over remapping to learn something about the as-ensemble properties of the water surface as a distorting/reimaging element.
The third previously mentioned earlier technology, dating to the 1960s, relates to processing of synthetic-aperture radar images. The noteworthy aspect of that work is simply that it provides an instance of a sort of image processing by analog (as distinguished from digital) computers.
No connection between any of these three earlier technologies and the observation/communication problems discussed in the preceding passages of this document has heretofore been drawn.
As can now be seen, in the field of the invention the prior art has failed to provide solutions to important difficulties of observing the operating environment and receiving communications.